In connection with enhanced integration and increased density of semiconductor devices, wires of circuits have recently become finer and finer, and the number of layers of multilayer interconnections has also increased. In order to realize multilayer interconnection while micronizing a circuit, it is necessary to precisely perform planarization processing on the surface of a semiconductor device.
CMP (Chemical Mechanical Polishing) is known as a technique of planarizing the surface of a semiconductor device. A polishing apparatus for performing CMP includes a polishing table to which a polishing pad is stuck, and a top ring for holding a polishing target (for example, a substrate such as a semiconductor wafer, or each kind of film formed on the surface of a substrate). The polishing apparatus polishes the polishing target by pressing the polishing target held by the top ring against the polishing pad while rotating the polishing table.
The polishing apparatus includes a film thickness measuring device to detect an endpoint of a polishing step based on the film thickness of a polishing target. The film thickness measuring device has a film thickness sensor for detecting the film thickness of the polishing target. An eddy current sensor is known as a representative of the film thickness sensor.
The eddy current sensor is disposed in a hole formed in a polishing table, and detects the film thickness of the polishing target when it faces the polishing target while rotated along with the polishing table. The eddy current sensor causes the polishing target such as a conductive film to induce eddy current therein, and detects variation of the thickness of the polishing target from variation of magnetic field occurring due to the eddy current induced in the polishing target. In order to use the eddy current sensor as a film thickness sensor, a calibration for obtaining a correspondence relationship between the film thickness and the measurement value of the eddy current sensor is necessary before an actual measurement is started.
The conventional calibration has been performed as follows. When the eddy current sensor is mounted in the polishing apparatus, a polishing pad is interposed between the conductive film as the polishing target and the eddy current sensor. When the thickness of the polishing pad varies, the output of the sensor also varies. In a prior art described in JP2007-263981A, the calibration is performed by using plural polishing pads having different pad thicknesses and plural calibration wafers having different film thicknesses. Since different pad thicknesses and different film thicknesses are combined with one another, measurement is performed a number of times to obtain a correspondence relationship between the film thickness of the polishing target and the measurement value of the eddy current.
The prior art has also the following problems. First, since the plural polishing pads for calibration are disposed on the polishing table, it is necessary to peel off a polishing pad which has already been stuck to the polishing table. Accordingly, one polishing pad must be discarded for the calibration, resulting in increase of the cost. Secondly, since a calibration wafer is manually placed on the polishing pad, there is a problem that the positional precision is low. Additionally, an error may occur, since dust or the like is attached to the polishing pad or the calibration wafer due to manual operation. Thirdly, since the calibration wafer is repetitively used, oxidation or the like occurs, so that deterioration of the calibration wafer may cause an error.